Mental-test scores and measures of school achievement often show a downward drift during childhood for low-SES children. Many reasons have been advanced to explain the decline, but the common refrain seems to be that detrimental qualities in the home environment often fail to promote mental development. There are, however, individual differences in terms of which child is more susceptible to these detrimental effects, or by contrast which one is better buffered and able to make substantial progress in spite of an impoverished environment. The role of intrinsic characteristics as modifiers of environmental impact would seem to be potentially important, although relatively little attention has been devoted to this topic. The primary goal of the proposed research is to recapture a sampel of low-SES twins (now inactive) who were originally recruited in 1972-75 for a longitudinal study of mental development. Most of the twins had completed a battery of mental tests from birth to 9 years before being terminated. These twins will be reaching their 12th birthday during 1984-86, and they will be reactivated for an assessment of intelligence, school achievement, and temperament at 12 years. A home visit has also been carried out for these families, which will furnish ratings of home and mother as effective agents for fostering mental development. A multivariate path analysis will be employed to identify the causal links between early mental development, home-and-maternal variables and the criterion measures obtained at 12 years. Twins also help clarify whether any role is played by perinatal risk and genetic factors in producing the range of individual differences.